The invention concerns a centering and adjusting contrivance for servo brakes in accordance with the precharacterizing clause of claim 1.
In one known adjusting and centering contrivance from DE-GM 72 19 84, for a slide-through lock of an inner shoe servo brake, we are dealing with an adjusting contrivance that is effective solely in one direction and which additionally includes a plate-formed adjustment lever having an adjustment pawl that meshes with a single sided toothing. In operation and upon incurring a slide-through motion of the slide-through lock, the adjustment lever is swivelled outwardly about an axis of rotation provided on the guide casing such that the arresting pawl first stops over one tooth. Thereafter, and with a subsequent centering of the slide-through lock effected by spring force, a resulting circumferential return movement causes the arresting pawl to mesh with the tooth previously stopped over and rotate an adjustment sleeve in the direction of screwing out the threaded bolt. Also provided for actuating the adjustment lever is an add-on piece capable of being moved with the slide-through lock and which cooperates with a stop on the adjustment lever for the purpose of swinging out. The reverse circumferential movement of the adjustment lever is effected via a return spring that engages on the one end to a transverse arm of the adjustment lever and that is anchored at the other end to the guide casing. The adjustment lever is secured to a bolt serving as a pivoting axis by means of a bracket clamp projecting through a transverse boring through the bolt.
In spite of its operability, however, this adjusting and centering contrivance displays serious disadvantages. For one thing, it has been shown in practice and in particular in the case of commercial vehicles with relatively high braking moments and with reversing operational application, that the operational safety at this type of adjusting mechanism no longer suffices for present-day requirements. Specifically and in the case of strong vibrations, it is possible with this type of adjusting mechanism that the adjustment lever may be induced to pivot outwardly by these vibrations and, thereby, without need, engage behind the next tooth of the adjustment gear and undertake to make an adjustment.
Furthermore, the return springs are also exposed to a certain amount of wear, so that, in the case of a comprehensive and intensive stressing, their return forces change. Additionally, it is further possible that as a slide-through movement of the slide-through lock induces the adjustment lever to pivot outwardly, the adjustment gear may move and follow along, so that meshing behind a next tooth is suppressed, thereby preventing the accomplishment of the desired adjustment. These disadvantageous actions occur in particular in the case of heavy commercial vehicles with a great amount of wear, and with strong impairment from dirt on the adjusting contrivance. Ultimately and finally, in the case of the previously known adjusting contrivance, the structural space that is stressed by the upwardly laterally extending lever arm, to which the return spring is attached, is relatively great.